Free Easy Anchor Rope/Chain Ratio Calculator

[svrevelations]

Sitting around at home, wishing we can be on our yacht and resume our world cruise, here is our Easy Anchor Rope/Chain Ratio Calculator in Acrobat Reader interactive (fill-able) format.

• Type in the anchor water depth.
• Add for your freeboard.
• Check the tide and add for high tide if required.
• Expecting a storm? Add for storm surge water height.
• The calculator provide you with 4 possible scope ratios.

svRevelations.com - Downloads

The Easy Anchor Rope/Chain Ratio Calculator is supplied as freeware and you can use it or modify it for your own needs. In addition, the Acrobat Reader file is not encrypted (password or otherwise) which means you can add further functionality if you have Acrobat with Form Designer.

[Jim Cate]

Well, that's a nicely done bit of software, but why in the world would someone need or even want to have to resort to their computer to do a very simple mental calculation, one that we have all been doing for years?

It isn't as if one needs four significant figures of accuracy for such things...

I applaud your workmanship and your generosity in making it available. I worry about the mind set that thinks it is a good thing to have.

Jim

[newhaul]

Originally Posted by Jim Cate

Well, that's a nicely done bit of software, but why in the world would someone need or even want to have to resort to their computer to do a very simple mental calculation, one that we have all been doing for years?

It isn't as if one needs four significant figures of accuracy for such things...

I applaud your workmanship and your generosity in making it available. I worry about the mind set that thinks it is a good thing to have.

Jim

Jim I have to agree with you . Great learning tool and new ppl using it will soon be able to figure it out just like we do. My freeboard is 3 ft.

[svrevelations]

Originally Posted by Jim Cate

Well, that's a nicely done bit of software, ................. set that thinks it is a good thing to have.

Jim

Point taken. I'm simply adding a bit of information to the vast pool of knowledge out there. As mentioned, it is mainly aimed at new people who has not done this before and might have questions in this regard. Also to the number of yacht operators we have seen anchoring and then end up with their yachts dragging anchor - some of them due to inadequate scope/ratio.

Use it if you wish or leave it if you know have no need.

[Island Time O25]

Originally Posted by Revelations

Point taken. I'm simply adding a bit of information to the vast pool of knowledge out there. As mentioned, it is mainly aimed at new people who has not done this before and might have questions in this regard. Also to the number of yacht operators we have seen anchoring and then end up with their yachts dragging anchor - some of them due to inadequate scope/ratio.

Use it if you wish or leave it if you know have no need.

I think it would make a great teaching tool as well as a part of emergency drill for nonsailing guests during longish sails. The little chart makes it easier to visualize for the nonsailors.

[Toph]

I agree with some of the others. Very nice work for some rather simple calculations. I think it would be nice to use as a 'big picture' check after a long days sailing. It is amazing how the most simplest of numbers can go astray when fatigued. So I downloaded it... Many thanks.

While there I also downloaded the crew list and float plan. Something I was going to do myself so a few hours saved there. So if ever we shall meet, I will cook you a BBQ form the cookbook I also download. Very neat and presentable work...

Thanks again.

[Stemci]

Thanks for sharing ⛵️

[groundtackle]

I Like it very much, well done. Good training tool for beginners. I would like to see it drop in scope increments right to zero or directly on top of the anchor, even more visually effective ( i did not download it yet, perhaps it does that already) and for times we all need to short scope. Nice to have something to double check when sleepy.
We are working on a nice little 'Reference Library' on our site right now. Would you give permission to place it there?
ce

[kas_1611]

Just downloaded and tried this but it does not calculate anything on my Mac. Not sure if this is a Mac Preview issue or not but thought I'd bring that problem up.

Also can I ask why (and where) you think that calm conditions require a 6:1 scope or 8:1 for normal, long term anchoring? Virtually all the anchoring information I have ever found uses 4:1 for chain under normal conditions, 5:1 for chain and rope and 7:1 for all rope. Yes you extend that for severe or extreme conditions but your scopes would cause havoc in most anchorages where many people (including myself) safely anchor in light winds with 3:1 down.

Believe me dropping 8:1 in most of the anchorages I've used in the Adriatic would have people screaming blue murder at you or you bumping into everything.

Do like the graphic representation of the scopes though, perhaps you could add the angle of pull at the anchor to really show why that little extra under severe conditions makes a big difference.

Keiron

[barnakiel]

I like how it works. Maybe you can add some small print info on what the rope/chain ratio is? Or at least how much chain there is in the shortest scope (as I assume there is only extra rope in the longer scopes).

Where we cruise now (West Indies) 6:1 is all but impossible in a number of popular places - simply too many boats there to get this much slack.

Great job. Thanks for sharing.

b.

[svrevelations]

Originally Posted by groundtackle

I Like it very much, well done. Good training tool for beginners. I would like to see it drop in scope increments right to zero or directly on top of the anchor, even more visually effective ( i did not download it yet, perhaps it does that already) and for times we all need to short scope. Nice to have something to double check when sleepy.
We are working on a nice little 'Reference Library' on our site right now. Would you give permission to place it there?
ce

It's freeware - you are welcome to use it as you please.

[svrevelations]

Originally Posted by kas_1611

Just downloaded and tried this but it does not calculate anything on my Mac. Not sure if this is a Mac Preview issue or not but thought I'd bring that problem up.

Also can I ask why (and where) you think that calm conditions require a 6:1 scope or 8:1 for normal, long term anchoring? Virtually all the anchoring information I have ever found uses 4:1 for chain under normal conditions, 5:1 for chain and rope and 7:1 for all rope. Yes you extend that for severe or extreme conditions but your scopes would cause havoc in most anchorages where many people (including myself) safely anchor in light winds with 3:1 down.

Believe me dropping 8:1 in most of the anchorages I've used in the Adriatic would have people screaming blue murder at you or you bumping into everything.

Do like the graphic representation of the scopes though, perhaps you could add the angle of pull at the anchor to really show why that little extra under severe conditions makes a big difference.

Keiron

Sorry for your difficulties. Not sure why it does not calculate on the Mac - perhaps it is something to do with Java Script. It works fine on Windows.

Regarding your scope questions/comments;
Clearly you have to use the scope ratios which you think is appropriate based on the area where you will anchor - situation, available space, conditions, etc. Although it is not always possible to anchor with 8:1, 10:1 or 12:1, we have done so on more occasions than not.

After anchoring, we do not always remain on the yacht and for this reason we are conservative about the scope. The last thing we desire is to return from an excursion and find the yacht dragged anchor - create damage to other yachts and/or itself sustaining damages. To date, we have never dragged anchor despite anchoring in very strong winds, tidal surges and even fast flowing rivers. Whilst you are welcome to follow your own calculations, the document reflects what we do when anchoring.

[svrevelations]

Originally Posted by barnakiel

I like how it works. Maybe you can add some small print info on what the rope/chain ratio is? Or at least how much chain there is in the shortest scope (as I assume there is only extra rope in the longer scopes).

Where we cruise now (West Indies) 6:1 is all but impossible in a number of popular places - simply too many boats there to get this much slack.

Great job. Thanks for sharing.

b.

I did not think about the chain/rope ratios due to our situation. We have 150 meters of chain spliced to another 250 meters of rope. Since it is one continuous chain/rope combination, we simply let enough meters inline with our calculations.

However, it is easy to implement the calculations you ask for. Let me know what the ideal rope/chain ratio is. I will be happy to add the calculations to the document and re-upload.

[Blaine]

Great job and thanks for sharing. Food for thought and food to eat... NICE!

As alluded to above, I'm curious as to rope/chain ratios as they relate to scope. I've heard a rule of thumb is that an all chain rode requires roughly half the length of an all rope rode to achieve about the same setting/holding properties.

I'm putting together new ground tackle now for my 42' trawler so this info is very timely and much appreciated. My cruising grounds in So. Cal. frequently entail 100' depths. I would love to have an all chain rode to use under "average" conditions with maybe 250 - 300' of rope as a backup if things got ugly.

I'd love to hear opinions as to how much chain to carry. (Chain will be 5/16 G-4 as required by windlass. Anchor will be 65 lb. Mantus.)

Thanks for all suggestions.

[MathiasW]

Anchoring is a very controversial topic, I know! Still, here I would like to put forward a free tutorial tool my son and I have created that allows one to estimate the maximal anchor load and the minimally required chain length after providing input parameters for the vessel as well as sea and weather conditions (i.e. swell and wind / gusts). It works with various physical units, like daN (deka Newton), kp (kilo pond, which is roughly a daN), or imperial units of pound and feet - the button on the top far right. Depending on which country you are coming from, the tool will try to make an intelligent guess to set the default correctly. The tool can be used in Basic Mode and Expert Mode. Advise is to start in Basic Mode and take it from there.

Have a look at the enclosed table, which shows the results for various anchoring scenarios. Wind and (large) swell is always the same and the maximally available chain is fixed at 50 metres. I chose three different anchor depths (measured from the bow, of course): 3, 5, and 9 metres. Finally, I varied the quality of the snubber / bridle - I define the quality by the snubber stretch at a given wind strength. First, if you look at 3 metres and no snubber, the tool gives an error, meaning that within my model the chain alone cannot cope with the (large) swell energy. Then, comparing 5 and 9 metres of anchor depth, one finds that, naturally, the pulling angle at the anchor shaft will slightly increase as the chain length is fixed, but at the same time the anchor load will go down from 1322 daN to 480 daN, which is less than half! This massive reduction of the anchor load more than compensates for the slight reduction in maximal holding power caused by the increased pulling angle. Finally, when you use an "excellent snubber", the anchor load gets further reduced to a mere 156 daN. This is less than 1/8 of the anchor load at 5 metres anchor depth and no snubber. What a difference! These results clearly show the importance of using excellent snubbers in shallow water in particular. Using more chain would not make much of a difference, as the chain is not effective in shallow water for absorbing shock loads.

Of course, in this analysis I did assume the swell to be the same at all anchor depths (and rather large), which is not in general true. But when it is, it is definitely safer to anchor in deeper water with that amount of swell present. (In my model, the vessel velocity at anchor is related to the swell energy.) Keeping all but one parameter fixed and only vary a single parameter is a typical scientific approach to study the effect this parameter has. So, please do not misunderstand this as me saying the swell is always the same, regardless of anchor depth. It certainly is not.

So, contrary to popular belief, it is not always better to seek the most shallow place to anchor. Experienced sailors will know this, but at least for me I did not know this when I started sailing - in my sailing classes there was no mentioning of this and the simple rule was: Use a scope of 3:1 in calm weather and more when it gets windy...

Here is the link to the online tool: https://anchorchaincalculator.com

If you do not like to read the tutorial nor watch the short tutorial video, here are some comments on the required inputs: Vessel type refers to whether it is a monohull, a cat, or a tri, and how its built is: slim, medium, or bulky. This information is required to estimate the Windage area of your vessel. Anchor depth is divided into two fields, which you can use as, e.g., freeboard and water depth, but any other split is fine as well. Vessel velocity at anchor is the maximally observed speed over ground in reverse direction away from the anchor, whilst at anchor. It is difficult to get this one precise, as chart plotters are not very precise, but often it is something in the range of 0.1 to 0.7 kn. This entry, together with Vessel weight determines the swell energy. As to Snubber quality - in Basic Mode I define it qualitatively in plain English words. Not scientific, I know, but the names do have a meaning - a good snubber is a good snubber, and a lousy one is a lousy one... If you want to dig deeper here, use the Expert Mode. For instance, an "Excellent snubber" has a stretch of 1.6 metres at 8 Beaufort wind strength, or 40.2 kn (in the absence of swell, which would come on top as a load). If you do happen to have the data sheet of your snubber line, you can use the Rule of Three to work out the input that the tool requires. If you use the qualitative descriptions of the snubber and find that the snubber stretch the tool calculates is very different from what you actually observe, you need to adjust the snubber quality rating you have given your snubber. My observation is that many people overrate their snubber initially - certainly I did when I started this... Obviously, a snubber of only 2 metres length cannot stretch by yet another 1.6 metres!

And when I say swell, I mean all kinds of waves hitting the vessel.

This tool is not meant to be used every time you anchor, but rather as a tutorial tool to play around and see how different scenarios pan out. For instance, it may allow you to make a first quick assessment of the quality of the snubber you are using. I hope it will help you to make better choices for anchoring more safely.

Feedback most welcome!

Cheers, Mathias

PS: If you want to understand the physics and model in more detail, in a nutshell, it is catenary plus a spring (the snubber) bolted onto it with matching loads. For yet more information, please visit: https://trimaran-san.de/die-kettenku...atiker-ankert/